1. Field of Invention
This invention relates generally to photonic integrated circuits (PICs) and more particularly to uncooled PICs operating as WDM transmitters or receivers and also particularly such PICs that operate with a floating wavelength grid of signal channels where the spatial separation between the signal channels is continuously maintained fixed.
2. Definitions
In order to better understand the disclosure, the following definitions are offered relative to certain terminology that is employed throughout this disclosure:
The term, “modulated sources”, includes directly modulated lasers and cw lasers with external modulators or any element or elements that provide a modulated signal at a given wavelength. The term, “external”, as used in the art in this context means “independent or separate from” the laser and the modulator is an integrated device on the same substrate with the laser.
Lasers or laser sources are the same element.
An “element”, which is synonymous with “component”, means any active or passive optical device integrated on a photonic integrated circuit (PIC) that performs a function on the PIC. Examples include, but not limited to, a laser, modulator, PCE, MFE, an element with a fixed insertion loss which can be fixed or set at a bias, a waveguide, a combiner or decombiner, a coupler, or splitter.
Reference to “WDM” is intended to include “DWDM” and “CWDM”. The term “channels” or “signal channels” has general reference to modulated sources on the PIC chip, such that if there are N signal channels, then there are N modulated sources. Reference in this description to circuit-integrated laser source/modulator signal channels is nominally a laser source and its associated modulator together comprising a modulated source and providing a modulated signal output. In this context, such a channel is also functions as an optical waveguide.
“Active region” as employed in the description in this application means the region in a semiconductor device where carrier recombination occurs which may be comprised of a single semiconductor active layer or multiple semiconductor layers with any associated optical confinement layers, as is well known to those skilled in the art.
A “channel” means an integrated, optical signal channel waveguide path in a single-channel EML or in a multiple channel PIC that minimally includes a modulated source or other active element, such as a photodetector (PD), for propagation of an optical signal and where, in the photonic integrated circuit (PIC), there are N signal channels formed in an array across the PIC where N is two or greater and or in an EML there is N=1.
A “combiner or decombiner” means a wavelength selective combiner or decombiner and a free space combiner or decombiner. A “wavelength selective combiner or decombiner” is a wavelength discriminating combiner or multiplexer of wavelength channel signals. A “free space combiner/decombiner” is a wavelength indiscriminate combiner with respect to combining different wavelength channel signals. More particularly, the output power, in units of dBs, from a wavelength selective combiner may be defined as
                    ∑        i        N            ⁢                          ⁢              P        i              -    IL    ,where Pi is the optical input power into the combiner, N is the number of outputs and IL is the insertion loss factor. For the free space combiner, it is typically defined as
                    ∑        i        N            ⁢                          ⁢                        P          i                ⁢                  /                ⁢        N              -    IL    ,where Pi is the optical input power to the combiner and N is the number of inputs and IL is the insertion loss factor. It can readily be seen that the difference power output is the prime difference. Examples of wavelength selective combiners/decombiners are, but not limited to, an arrayed waveguide grating (AWG), an Echelle grating, a cascaded Mach-Zehnder interferometers, a quasi-wavelength selective star coupler or an elliptical supergrating. Examples of free space combiners/decombiners are, but not limited to, a multimode interference (MMI) coupler, free space coupler, star coupler or any such optical coupler with a multimodal coupled region.
As employed in this description, a photonic integrated circuit (PIC) may be any semiconductor device, including a silicon device, which has at least two elements integrated in an optical circuit. Thus, a PIC can be an EML, TxPIC, RxPIC or any other circuit with a plurality of elements, passive or active, formed in the circuit.
“Laser emission wavelength” means emission output wavelength of a laser or lasers.
“Active region wavelength” means the wavelength of the photoluminescence peak or the gain peak in am active region of element of in a signal channel formed in a photonic integrated circuit (PIC), such as, for example, an active region of a laser source, a modulator, a monitoring element such as a photodetector or an power changing element (PCE) such as a semiconductor optical amplifier (SOA), a variable optical attenuator (VOA) or a variable gain/loss element such as a combination SOA/VOA device.
“Spectral spacing (Δλ)” means the difference in laser emission wavelengths between adjacent signal channels in a photonic integrated circuit (PIC) or between discrete modulated sources.
“Laser detuned offset” means the difference between laser emission wavelength and the laser active region wavelength for a signal channel.
“Positive wavelength detuning” or “positively detuned wavelengths” means the laser detuned offset that is greater than zero. As used herein, “positive wavelength detuning” can also include slightly negative wavelength detuning, i.e., just below zero, since in fabricating PICs, it is possible that intended positively detuned wavelengths can end up slightly negatively detuned by a few nanometers.
“Laser-modulator detuning” means the difference between the laser emission wavelength, and the modulator active region wavelength relative to the same signal channel.
“Operation window” means the range of laser source channel emission wavelengths over which there is acceptable loss and acceptable bit error rate (BER) performance of the modulated sources for a particular specified application of the PIC.
“PCE” means a power changing element (a power varying element or a fixed loss element) integrated in one or more of the channels of a photonic integrated circuit (PIC) that changes the power level of the light propagating through the element. Examples of PCEs are photodetectors, semiconductor optical amplifiers (SOAs), variable optical attenuators (VOAs), or combination SOAs/VOAs which may also be referred to as ZOAs, Δ-β coupler, a Mach-Zehnder interferometer that changes the phase of light split between the interferometer arms, or the deployment of an absorption region of a predetermined length formed in the signal channel.
The wavelength grid of a plurality of modulated sources as well as the wavelength grid of a combiner or decombiner is also referred to as a “wavelength comb”.
A “slew rate” is defined as a rate related to how rapidly the wavelength grid moves or changes in frequency in a coolerless (heated) ambient, which can be measure in nm/° C. or GHz/° C. As an example, on a TxPIC described herein, the laser sources slew rate is around 16.5 GHz/° C. such as in an ambient between approximately room temperature and approximately 70/° C.